Enable C11 support

This uses the gnu11 standard, which is C11 with GNU extensions.
Previously, gnu89 was being used, which is pretty ancient by now. C99
brings some important improvements, some of which were already available
and used even without this option. C11 is more recent and brings more
minor improvements. Most notable feature is the static_assert statement,
allowing checking invariants at compiletime using the full C
expressions.

Author: matthijskooijman

platform.txt

@@ -19,7 +19,7 @@ compiler.warning_flags.all=-Wall -Wextra
 
 compiler.path={runtime.tools.arm-none-eabi-gcc.path}/bin/
 compiler.c.cmd=arm-none-eabi-gcc
-compiler.c.flags=-c -g -Os {compiler.warning_flags} -std=gnu89 -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500 -Dprintf=iprintf -MMD
+compiler.c.flags=-c -g -Os {compiler.warning_flags} -std=gnu11 -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500 -Dprintf=iprintf -MMD
 compiler.c.elf.cmd=arm-none-eabi-gcc
 compiler.c.elf.flags=-Os -Wl,--gc-sections
 compiler.S.cmd=arm-none-eabi-gcc